The conventional stereo method pays attention to a specific pixel on a basis image and then performs depth estimation by search a corresponding point of the specific pixel on a reference image. Three-dimensional information of the target object that is obtained by such a method, is a point group in the three-dimensional space and does not directly represent a dense surface geometry of the target object (hereinafter also simply referred to as “a three-dimensional surface” or “a surface”).
That is to say, in the conventional stereo method, there is a problem that a correlation between adjacent pixels in an image and points in the three-dimensional space corresponding to the adjacent pixels is not made.
Further, an approach for generating a three-dimensional surface of a target object by using stereo images, is a method that the depth data obtained by the stereo method is utilized, i.e. after obtaining the surface of the target object as a set of points, the points in the three-dimensional space are applied to a surface model (for example, see Non-Patent Document 1, Non-Patent Document 2, Non-Patent Document 3 and Non-Patent Document 4). Conventionally, such an approach has been studied actively as a technology that utilizes three-dimensional information obtained by the stereo method as well as a laser range finder, and so on.
However, the information of points in the three-dimensional space obtained by the conventional stereo method, includes errors as a result of having been affected greatly by image noises etc., so there are various problems such as it is impossible to generate a proper surface, it is necessary to perform a complicated computation, in the method that generates a surface of a target object from the three-dimensional information having such big errors. Further, since the most of these methods base on interpolating and modifying the original three-dimensional information, there is a problem that the luminance information of stereo images is not utilized effectively.
In order to solve such a problem, an approach that directly generates the surface geometry of the target object by using the luminance information of stereo images, is proposed. As such a method, such as, there are a method that generates polygon meshes from stereo images (see Non-Patent Document 5) and a method that optimizes oriented particles (see Non-Patent Document 6).
However, since these methods are aimed at obtaining an accurate three-dimensional model of a still object such as a sculpture or a doll, in general, the computation time of around several minutes is necessary. In the case of applying such methods to the navigation of such as a robot or an automobile, a problem that the computation time is long, occurs. For example, for a robot that moves, it is necessary to immediately obtain shapes (geometries) of such as the gradient of a forward slope and a forward step, and further, at the time of a landing of a rocket or a helicopter, it is necessary to immediately obtain a ground surface.
Furthermore, as a method that directly generates a three-dimensional surface of an object from stereo images, there is a method that directly generates a three-dimensional surface geometry of a target object from stereo images by using a method to be called a level set method (for example, see Non-Patent Document 7). This method is an approach that the surface geometry of an object is represented by a function and then a differential equation is solved by iterative computations. However, since the iterative computations need large computational complexity, there is a problem that it is impossible to generate a three-dimensional surface of a target object in a short time.